Thanks Dhiman and Alistu for replying,
now I increase the bit rate from 240-Mb/s to 24 Gb/s, increase the length of SMF from 5Km to 100 Km and decreases the input power of the laser from 0d dBm to -5 dBm.
Still the BER is zero and now the Q-factor is in thousands.
I think there is a problem with the configured parameters or I am using wrong components for the system.
Kindly help me in this matter.
Regards